Required a number in text box in Inno Setup

You can setup the next button to be enabled or disabled in the CurPageChanged event when the user reaches the page where resides your edit box. Except that you need to monitor changes of that edit box to enable or disable the next button according to whether there's something entered in that edit box. For this you need to write a handler for the OnChange event. Here is an example:

[Setup]
AppName=My Program
AppVersion=1.5
DefaultDirName={pf}\My Program

[Code]
var
  MyEdit: TNewEdit;
  MyPage: TWizardPage;

procedure MyEditChange(Sender: TObject);
begin
  { enable the next button if the edit box is not empty; disable otherwise }
  WizardForm.NextButton.Enabled := MyEdit.Text <> '';
end;

procedure MyEditKeyPress(Sender: TObject; var Key: Char);
var
  KeyCode: Integer;
begin
  { allow only numbers }
  KeyCode := Ord(Key);
  if not ((KeyCode = 8) or ((KeyCode >= 48) and (KeyCode <= 57))) then
    Key := #0;
end;

procedure InitializeWizard;
begin
  MyPage := CreateCustomPage(wpWelcome, 'Caption', 'Description');

  MyEdit := TNewEdit.Create(WizardForm);
  MyEdit.Parent := MyPage.Surface;
  MyEdit.Left := 0;
  MyEdit.Top := 0;
  MyEdit.Width := 150;
  MyEdit.OnChange := @MyEditChange;
  MyEdit.OnKeyPress := @MyEditKeyPress;
end;

procedure CurPageChanged(CurPageID: Integer);
begin
  { if the currently turned wizard page is the one with the edit box, enable }
  { the next button if the edit box is not empty; disable otherwise }
  if CurPageID = MyPage.ID then
    WizardForm.NextButton.Enabled := MyEdit.Text <> '';
end;

There are two flaws in the @TLama's answer:

• A user can bypass the check by using Paste command from edit box context menu (and possibly by other kinds of input methods).

  To fix this, you can introduce a last resort check in NextButtonClick.

  I would also suggest adding the check for an empty input there, instead of MyEditChange, as it allows providing feedback to the user, explaining what is wrong.

• On the other hand, Ctrl+C, Ctrl+V and Ctrl+X keys are not working. Particularly a lack of Ctrl+V is not comfortable.

  To fix this, allow these control characters in the MyEditKeyPress.

[Code]

var
  MyEdit: TNewEdit;
  MyPage: TWizardPage;

procedure ValidateMyEdit;
begin
  { enable the next button if the edit box is not empty; disable otherwise }
  WizardForm.NextButton.Enabled := (MyEdit.Text <> '');
end;  

procedure MyEditChange(Sender: TObject);
begin
  ValidateMyEdit;
end;

function IsDigit(C: Char): Boolean;
begin
  Result := (C >= '0') and (C <= '9')
end;

procedure MyEditKeyPress(Sender: TObject; var Key: Char);
begin
  if not ((Key = #8) or { Tab key }
          (Key = #3) or (Key = #22) or (Key = #24) or { Ctrl+C, Ctrl+V, Ctrl+X }
          IsDigit(Key)) then
  begin
    Key := #0;
  end;
end;

procedure InitializeWizard;
begin
  MyPage := CreateCustomPage(wpWelcome, 'Caption', 'Description');

  MyEdit := TNewEdit.Create(WizardForm);
  MyEdit.Parent := MyPage.Surface;
  MyEdit.Left := 0;
  MyEdit.Top := 0;
  MyEdit.Width := 150;
  MyEdit.OnChange := @MyEditChange;
  MyEdit.OnKeyPress := @MyEditKeyPress;
end;

procedure CurPageChanged(CurPageID: Integer);
begin
  if CurPageID = MyPage.ID then
  begin
    ValidateMyEdit;
  end;
end;

function NextButtonClick(CurPageID: Integer): Boolean;
var
  I: Integer;
begin
  Result := True;

  if CurPageID = MyPage.ID then
  begin
    for I := 1 to Length(MyEdit.Text) do
    begin
      if not IsDigit(MyEdit.Text[I]) then
      begin
        MsgBox('Only numbers are allowed', mbError, MB_OK);
        Result := False;
        Break;
      end;
    end;
  end;
end;